That’s when I noticed the other O₂ tank.

The Hab has two reserve O₂ tanks. One on each side of the structure, for safety reasons. The Hab can decide which one to use whenever it wants. Turns out it’s been topping off the atmosphere from Tank 1. But when I add O₂ to the system (via the oxygenator), the Hab evenly distributes the gain between the two tanks. Tank 2 has been slowly gaining oxygen.

That’s not a problem. The Hab is just doing its job. But it does mean I’ve been gaining O₂ over time. Which means I’m not consuming it as fast as I thought.

At first, I thought “Yay! More oxygen! Now I can make water faster!” But then a more disturbing thought occurred to me.

Follow my logic: I’m gaining O₂. But the amount I’m bringing in from outside is constant. So the only way to “gain” it is to be using less than I thought. But I’ve been doing the hydrazine reaction with the assumption that I was using all of it.

The only possible explanation is that I haven’t been burning all the released hydrogen.

It’s obvious now, in retrospect. But it never occurred to me that some of the hydrogen just wouldn’t burn. It got past the flame, and went on its merry way. Damn it, Jim, I’m a botanist, not a chemist!

Chemistry is messy, so there’s unburned hydrogen in the air. All around me. Mixed in with the oxygen. Just… hanging out. Waiting for a spark so it can blow the Hab up!

Once I figured this out and composed myself, I got a Ziploc-sized sample bag and waved it around a bit, then sealed it.

Then, a quick EVA to a rover, where we keep the atmospheric analyzers. Nitrogen: 22 percent. Oxygen: 9 percent. Hydrogen: 64 percent.

I’ve been hiding here in the rover ever since.

It’s Hydrogenville in the Hab.

I’m very lucky it hasn’t blown. Even a small static discharge would have led to my own private Hindenburg.

So, I’m here in Rover 2. I can stay for a day or two, tops, before the CO₂ filters from the rover and my space suit fill up. I have that long to figure out how to deal with this.

The Hab is now a bomb.

CHAPTER 5

LOG ENTRY: SOL 38

I’m still cowering in the rover, but I’ve had time to think. And I know how to deal with the hydrogen.

I thought about the atmospheric regulator. It pays attention to what’s in the air and balances it. That’s how the excess O₂ I’ve been importing ends up in the tanks. Problem is, it’s just not built to pull hydrogen out of the air.

The regulator uses freeze-separation to sort out the gasses. When it decides there’s too much oxygen, it starts collecting air in a tank and cooling it to 90 kelvin. That makes the oxygen turn to liquid, but leaves the nitrogen (condensation point: 77K) still gaseous. Then it stores the O₂.